The concept is similar to a jackshaft arrangement, like the Starling pictured below on the left, but this time both chains are on the drive side of the bike. This means a regular crank can be used (fitting a standard crank the wrong way around with the chainring on the left-hand side can cause the pedals to unthread while riding) and, crucially, means it can also work with mid-drive e-bike motors.
Most high-pivot bikes these days instead use a single chain but with an idler pulley above the chainring, either on the mainframe or swingarm, so that the upper chain line is roughly aligned with the main (or effective) pivot point of the suspension (such as the Trek Session above on the right). Compared to this layout, Dave’s design has a few claimed advantages.
The first is chain wrap and reliability. For packaging reasons, many idler-drive bikes tend to use small idler wheels (the Trek pictured above uses an idler with only 13 teeth, but 15-16 teeth pulleys are common). The smaller the sprocket, the more pressure on the teeth and so the faster the sprocket wears out. Dave’s design uses sprockets of at least 16 teeth to reduce tooth pressure and sprocket wear. The primary chain (connecting the crank to the first driven sprocket) can have odd-numbered teeth to reduce chain wear, or else use a half-link chain or a belt, which Dave claims could last the lifetime of the frame.
Then there’s chain wrap. The patent states that, following best practice engineering principles, the chain should ideally wrap around a sprocket for at least 120° to avoid excessive wear or chain skipping. In order to achieve this with an idler drive, there usually needs to be a lower pulley on the slack (lower) side of the chain, just behind the chainring. These pulleys can be necessary anyway to reduce chain growth in the lower chain span as the suspension compresses, especially for bikes with very high pivot locations or long suspension travel. So while Dave’s design looks more complicated, there are the same number of sprockets in contact with the chain as there are in a conventional high-pivot with an upper and lower pulley (three – not counting the derailleur and cassette).
Furthermore, because the sprockets in this patent are larger than conventional idler pulleys, it could be more efficient. That’s because as the sprocket gets smaller the angle through which the chain has to bend at each link as it rolls onto and off of the sprocket (known as the articulation angle) increases dramatically – see the graph below on the right.
According to the patent, the total angle through which each chain link has to articulate back and forth as it snakes through the drivetrain is less in Dave’s design when compared to an idler with a lower pulley design, which, it says, could lead to less energy lost in the chain pins and so better efficiency – “Engineering all sprockets other than those in the cassette to have 16 or more teeth can provide increased efficiency over typical idler drive cycles which feature slack side and tension side idler sprockets of 15 and fewer teeth.”
The patent provides some examples of sprocket configurations for this and conventional idler drivetrains and compares the total amount of chain articulation involved in each.
Finally, in a conventional drivetrain, the gearing is determined by the chainring; for most 29ers, the most common size is 32-tooth. By using a smaller first-driven sprocket than the two driving sprockets, it is possible to achieve the same gearing ratio with a smaller sprocket on the crank (22-30 teeth according to this table of workable sprocket combinations). This has two advantages: one is to increase ground clearance and the other is to increase the amount of critical space between the tire, the chainring, and the chainstay yoke.
It’s mostly not about performance
When I called Dave Weagle to discuss this patent, he told me that the performance advantages were “less than 50%” of its value”. The rest is on the manufacturing side. According to the patent, “packaging issues surrounding clearance between rear tires and chainstay yokes now drive close to 70% of the total time spend developing a new frame model.” Using a smaller chainring at the crank, along with elevating the swingarm, frees up valuable space in this critical area. This real estate could be used to make simpler chainstays with no need for complex forgings, carbon moldings, or even yokes. The swingarm could therefore be lighter, cheaper, and stronger.
Alternatively, it could be possible to design bikes with shorter chainstays. This is not something everyone wants, but with a 29″ wheel the shortest achievable chainstay length is about 430 mm, and smaller riders might benefit from shorter chainstays to allow them to get their weight over the rear axle when they want to manual. Moreover, the high-pivot design makes it easier and cheaper to vary the chainstay length without having to change the swingarm or the suspension kinematic. So it could be easier to have size-specific chainstay lengths, including very short stays for the smallest size.
The design could work with e-bike motors too, and with many of the same components.
The case gets stronger with e-bikes because space behind the BB is even more limited and long chainstays are more problematic due to the added weight of the battery in front of the bottom bracket. Plus, because the swingarm and main pivot are up and out of the way of the motor, it may be possible to share hardware (including the pivots, sprockets, and swingarm) between e-bikes and regular bikes. This too could help bike companies keep costs down.
How high is “high pivot”?
While some high pivot advocates wax lyrical about the benefits of fully rearward axle paths, the patent advocates a more moderate approach: “By tactically placing the inflection point [that’s the part of the travel where the axle path stops moving backward relative to the mainframe and starts moving forward] between 40% and 80% of the vertical rear wheel displacement, as is possible in the disclosed assemblies, a more stable horizontal chainstay length can be maintained.”
On our call, Dave explained that he thinks the horizontal chainstay length should be as consistent as possible during the part of the travel used during cornering (40-80% travel), so the load on the rear tire (which determines grip) is as consistent as possible too.
In other words, he’s not so much advocating a rearward axle path but a vertical one, especially in the middle of the travel where cornering grip is most critical. Otherwise, he says, if the axle path moves forwards or backward too much, the load on the rear tire changes as the suspension moves in and out of its travel, leading to inconsistent grip. This is simply because the closer the rear axle is to the bottom bracket, the higher the percentage of the rider’s weight is supported by the rear wheel.
The chart opposite shows the axle path of the “mid-high pivot” 2022 Trek Session; it’s roughly vertical overall, with an inflection point at around 50% travel. Alongside it on the right is the axle path of the idler-free 2018 Session 29, which has an overall forward axle path. Note the horizontal axis is magnified, but with the idler-free bike, the horizontal chainstay length will shorten by as much around 20 mm as the suspension compresses, which will shift load towards the rear wheel.
It’s true that the front wheel has a highly rearward axle path, meaning the front center shortens as the fork compresses, and this means that to maintain a constant weight distribution when the front and rear suspension compress in unison, you’d actually want a forwards axle path. But Dave points out that the fork and shock rarely compress in phase, so you may as well have the most consistent chainstay length you can (aka a vertical axle path).
Will it see production?
When I talked to him, Dave wasn’t over-selling this idea. “Do I think that 150 mm bikes will be running around with this technology? They could… but I’m not going to sit here and tell you it’s going to be some superior riding thing,” he said.” “It does work and it solves a bunch of issues. It has its small range of places where I think it’s applicable and useful… We’ll see if it goes to market – I hope it does.”
